1. Field of The Invention
The present invention relates to a displacement detector utilizing changes of capacitance, and more particularly to a displacement detector utilizing changes of capacitance which detects a moving amount of a moving element with the basis of the changes of capacitance between both electrodes of moving moving electrodes linked with the moving element and stator electrodes fixed to the basement.
2. Description of The Prior Art
Conventionally, there is the well-known displacement detectors which electrically convert the mechanical displacement of a probe or a moving stage and detect the displacement amount. Ordinarily, this kind of apparatus includes a moving moving element movably equipped with the basement and an encoder detecting the moving amount of the moving element and converting this into electric signal pulse to output therefrom, counts the electric signal pulse output from the encoder at a counter circuit, and displays the counted value on the digital display unit in digital representation. For the encoder to be used for this kind of apparatus conventionally well-known are the photo-electric type encoder, the mechanical contact type encoder, capacitance type encoder, etc.
The photo-electric type encoder includes slits prepared on the surface of a scale or a rotor disc at an equal distance and an emitter and a receiver which form a light path through the slits of the scale or the rotor disc, and detects the displacement amount of the moving element in accordance with the displacement amount of the moving element by means of the on and off operation of the light path formed between the emitter and the receiver by the movement or the rotation of the scale or the rotor disc.
However, this photo-electric type encoder has such drawbacks that the large power consumption in the emitter increases replacing times of batteries in use and the apparatus becomes large sized in total if large capacity batteries are used. Furthermore, in order to increase the measuring accuracy it is necessary to prepare the slits on the scale or the rotor disc at a few micron interval, which causes in manufacturing problem. There is another problem that the clearance change during the apparatus in operation causes in miscount.
The mechanical contact type encoder uses slits and brush to detect the displacement amount of the moving element, and the slits and brush wear very fast. There is also another problem that noises are mixed into the measurement signal.
On the contrary, the capacitance type encoder is widely used in the detecting device of moving element in recent years since this capacitance type encoder is of no large power consumption as the photo-electric type encoder and has no slits and brush either to cause the noises in the mechanical contact type encoder.
Conventionally, in the capacitance type encoder used for the displacement detector, plural pairs of electrode plates are facingly arranged to from capacitors so that both of electrode plates can be relatively moved in accordance with the displacement amount of the moving element, and the mechanical movement amount is electrically detected as change of capacitance in the capacitors.
For example, the electrode plates are arranged on the main scale in plural numbers at an equal interval, and the other electrode plates are facingly arranged on the index scale with a certain distance from the main scale. The main scale or the index scale is slidingly moved inaccordance with displacement of the moving element in parallel with the surfaces of the plates so that the apparatus can detect the displacement amount of moving element by the capacitance of the capacitor formed by both electrode plates.
The capacitance type encoder in the prior art device uses the capacitors consisting of the moving electrode plates mentioned above to form a voltage dividing circuit and detects the displacement amount of the moving element by means of detecting voltage dividing ratio changing inaccordance with capacitance of the capacitors. Accordingly, in the prior art device, in case the distance between the surfaces of the moving electrode plates forming capacitors changes for some reason to change the capacitance of the capacitors and the power voltage applied to the voltage dividing circuit changes, the divided voltage output does not respond with the displacement amount of the moving element with accuracy and the accurate measurement cannot be performed.
In order to solve the problems of the prior art device mentioned above, such a displacement detector utilizing changes of capacitance has offered that one side electrodes of electrode pairs consisting of plural pairs are respectively applied alternating current voltage with different phases to detect voltage signal induced in the other side electrodes, and that the relative moving amount is obtained by means of detecting the phase changes of the output signal changing with the basis of the relative movement of both electrodes against the standard phase.
FIGS. 1 and 2 are illustrations showing the composition of the electrodes in the displacement detector utilizing changes of capacitance thus offered On the surface of a stator plate 10 consisting of scale plate, etc. fixed to the base, as shown in FIG. 2, a plurality of transmitting electrodes are arranged at equal distances, and a receiving electrode 14 is zonally arranged in parallel with these transmitting electrodes 12. Each of the transmitting electrodes 12 mentioned above is applied alternating current voltage signal with different phases.
A moving plate 16 is facingly arranged to the stator plate 10 and consists of a rotor linked with the moving element or a movable scale plate. On this moving plate 16 connecting electrodes 18 facingly arranged in the spanning state over to both of the transmitting electrodes 12 and the receiving electrode 14 and earth electrodes 20 facingly arranged in the spanning state over the transmitting electrodes 12 and the receiving electrode 14 are alternatively arranged along with the moving direction of the moving plate 16.
On the other hand, the voltage signal is induced in the receiving electrode 14 in accordance with the voltage signal of the respective transmitting electrodes 12 mentioned above by way of the connecting electrodes 18. Therefore, when the moving element is displaced in such state that alternative current with different phases is applied to each of the transmitting electrodes 12 mentioned above, the output signal with the phase in accordance with the displacement amount of the moving element can be obtained from the receiving electrode 14. Accordingly, the integral process of the phase in the output signal from this receiving electrode 14 by an integrator 15 and the further comparison with the predetermined standard phase enables the displacement amount of the moving element be measured with accuracy without influence of fluctuating power voltage.
In recent years, in order to increase the portable and operational capability, the apparatus has been requested to be designed in small sizes. In order to design the prior art device in small sizes, each of the electrodes must be designed in smaller area, and distance between the electrodes must be more narrow. Because of the above mentioned, the signal output voltage from the receiving electrode 14 decreases and signal to noise ratio takes decreases by noise mixture which is made from the input portion of the transmitting electrodes 12 not through the connecting electrodes 18 due to the narrow distance between the electrodes. The lowering phenomenon of signal to noise ratio makes the measuring accuracy of the device decrease.
Ordinarily, among a pluraity of transmitting electrodes 12, the input terminal pattern is arranged on the reverse side of the stator plate 10 to commonly connect the mutual transmitting electrodes which the same phase alternating current voltage is applied to. The conductive paths of this input terminal pattern are plurally arranged in parallel with the arranging direction of the transmitting electrodes 12 with the basis of the respective phases.
The design of the device in small sizes makes the electrode area of the respective electrodes be smaller and the distance between the electrodes be more narrow. Because of this, the signal output voltage of the receiving electrode 14 decreases, and the noise mixture lowers signal to noise ratio.
In order to prevent such noise mixture, the device can be composed as the conductive paths of the input terminal pattern can be arranged so that the conductive paths having the inverted phase can be adjacent to each other and the noise in one conductive path can be mitigated or cancel the noise in the other conductive path.
However, since the distance between the receiving electrode and the respective conductive paths is different in the respective phases, the noise mixed from the respective conductive paths into the receiving electrode is so strong or weak that the receiving electrode cannot be completely prevented from the noise mixture even if the conductive paths having the inverted phase are adjacently arranged as mentioned above, and there still exists such a problem that the device cannot obtain the highly accurate detecting capability.